alt Hacker NewsPresumably this is data taken from interdicted phones of people in the groups, not, like, a traffic-analytic attack on Signal itself.
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I wonder whether the protesters could opt for offshore alternatives that don't require exposing their phone number to a company that could be compelled to reveal it by US law. For example, there is Threema[1], a Swiss option priced at 5 euros one-time. It is interesting on Android as you can pay anonymously[2], therefore it doesn't depend on Google Play and its services (they offer Threema Push services of their own.) If your threat model includes traffic analysis, likely none of it would make much difference as far as US state-side sigint product line is concerned, but with Threema a determined party might as well get a chance! Arguably, the US protest organisers must be prepared for the situation to escalate, and adjust their security model accordingly: GrapheneOS, Mullvad subscription with DAITA countermeasures, Threema for Android, pay for everything with Monero?
It appears to be primarily getting agents into the chats. To me the questionable conduct is their NPSM-7-adjacent redefining of legal political categories and activities as "terrorists/-ism" for the purpose of legal harassment or worse. Whether that is technically legal or not it should be outrageous to the public.